1. Field of the Invention
This invention relates generally to phase shifters utilized, for example, in electronically scanned phase array antennas, and particularly to phase shifter circuits incorporating low loss, RF microelectromechanical (MEMS) switches.
2. Description of the Related Art
The beam of a multiple element or array antenna may be propagated at a predetermined angle by inserting an appropriate phase shift in the radiated signal at each element of the array.
FIG. 1 is a simplified diagram of one row of a conventional phased array antenna 10 utilizing electronic beam steering, a complete planar phased array antenna having a number of such rows. The antenna 10 includes a plurality of radiating elements 12 each of which has its own phase shifter 14. An input line 16 carrying a transmission signal is coupled to each phase shifter 14, which imparts a respective predetermined phase shift (φ, 2φ, 3φ and 4φ, respectively) to the transmission signal as it passes through that phase shifter. The phase shifted transmission signals are then coupled to respective radiating elements 12 for propagation of the beam. Various types of phase shifters 14 have been developed, including switched-line phase shifters, reflection-line phase shifters and loaded-line phase shifters.
An example of switched-line phase shifters is the true time delay (TTD) phase shifter circuit in which rapid phase changes for electronically scanning the beam are obtained by selectively inserting and removing discrete lengths of transmission lines by means of high speed electronic switches. For example, with a cascaded switch arrangement, a relatively small number of preselected transmission line lengths can be series-connected in various combinations to provide a substantial number of discrete delays. Thus, a cascaded four-bit switched phase shifter can insert sixteen different phase shift levels into the propagated signal.
By virtue of their superior isolation and insertion loss properties, RF MEMS switches are advantageous for implementing high performance, electronically scanned antennas. However, conventional MEMS-based TTD phase shifters employ monolithic architectures that present processing compatibility, cost and packaging problems. For example, although most of the monolithic die area simply comprises easily fabricated passive metal delay lines, a monolithic architecture requires processing of the entire phase shifter circuit through a series of complex, multi-level MEMS switch fabrication steps. This not only results in low yields and high product costs, but as a result of incompatibilities between the delay line and MEMS switch fabrication processes, also restricts the materials that can be used.